Fixes #21 — Curated YouTube videos per guide + grammar item

Each of the 56 tense guides and grammar notes gets a curated YouTube video
attached (54 with picks, 2 silent nulls on rare / hard-to-find topics).
Users can stream in YouTube/Safari, download via YouTubeKit for offline
viewing, or play the local MP4 full-screen via AVPlayer.

YouTubeVideoStore loads the bundled youtube_videos.json at launch and serves
lookups by tense id or grammar note id. VideoDownloadService resolves the
best progressive MP4 stream off the main actor (YouTubeKit isn't Sendable),
writes to documents/videos/<videoId>.mp4, and records a DownloadedVideo row
in the local SwiftData container so the app knows what's on disk across
launches.

VideoActionsButtonRow is the unified UI for both detail views: three large
buttons — Stream (red, always enabled), Download (blue, disabled while in
flight and after completion, shows progress), Play (green, enabled only
when downloaded). Full-screen cover on tap. Settings gains a Downloaded
Videos list with swipe-delete, total-size summary, and a 500 MB warning.

Local store reset version bumped to 4 for the new DownloadedVideo schema.

Known fragility: YouTubeKit scrapes YouTube's private stream API and will
break when YouTube changes their internal format. Streaming keeps working
regardless.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -0,0 +1,15 @@
+{
+  "originHash" : "1b6ada17bf1104878f9520a6f7cb3cd84338c0da74dc3761cef075709d7df45d",
+  "pins" : [
+    {
+      "identity" : "youtubekit",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/alexeichhorn/YouTubeKit.git",
+      "state" : {
+        "revision" : "65be95dbb1dbd749499e0638871568c823822276",
+        "version" : "0.4.8"
+      }
+    }
+  ],
+  "version" : 3
+}